The present invention relates to non-magnetic black particles, a non-magnetic black pigment therefrom and a non-magnetic black filler therefrom. More particularly, the present invention relates to non-magnetic black particles having excellent blackness, heat resistance and dispersibility; a non-magnetic black pigment comprising the non-magnetic black particles; a non-magnetic black filler comprising the non-magnetic black particles; a paint containing the non-magnetic black pigment for forming a coating film having a high acid resistance; a rubber or resin composition containing the non-magnetic black pigment and having a high aging resistance; and a magnetic recording medium containing the non-magnetic black filler and having not only an excellent durability but also a high signal recording property.
As typical black pigments, there are known magnetite particles, carbon black, graphite fluoride or the like. These black pigments have been extensively utilized, for example, as coating materials which are prepared by mixing and dispersing the pigments in a vehicle, colorants which are prepared by mixing and dispersing the pigments in rubber or resin, or the like.
In recent years, from the standpoints of safety and hygiene and improvements in properties or in working efficiencies to meet requirements of energy-saving, there has been a strong demand for safe and non-noxious black pigments which are excellent not only in blackness and heat resistance but also in dispersibility, so that the workability thereof becomes excellent.
Conventional black pigments such as magnetite particles, carbon black and graphite fluoride have an excellent blackness, for example, a coating film formed by using such black pigment has a hue as represented by L* value of 10 to 25, a* value of -2.5 to 2.5 and b* value of -2.5 to 2.5.
In the case where these black pigments are incorporated into rubbers or resins to be molded, it is required that the pigments have a high heat resistance because they are inevitably exposed to an elevated temperature in the molding processes.
Also, in the case where these black pigments are contained in coating films to be subjected to baking finish at a high temperature, it is required that the pigments are free from discoloration even when heated at an elevated temperature such as 100 to 400.degree. C., though the temperature range is varied depending upon kinds of resins used in the coating films.
Further, in order to improve the workability of these black pigments, it is important that the pigments are provided in the form of non-magnetic particles which have adequate particle sizes, an excellent dispersibility in a vehicle, and good handling properties.
The use of such black pigments having a good dispersibility allows coating films in which the pigments are dispersed, to have a clear color tone. This results in not only the improvement in basic characteristics inherent to pigments, such as tinting strength, hiding power or the like, but also enhanced gloss and clarity of the coating films. In addition, if the black pigments having such improved dispersibility are used, film properties of the resultant coating films, such as acid resistance and aging resistance, can also be considerably improved. Accordingly, it is strongly required to provide black pigments having a higher dispersibility.
Further, in association with recent deterioration in natural and human-life environments such as acid rains, it has also been required to improve acid proof (acid resistance) and aging resistance of film-forming materials and rubber or resin compositions, especially those used outside.
Nevertheless of safe and non-noxious substance, the magnetite particles have still posed a problem concerning heat resistance, because the black-colored magnetite particles initiates to be transformed into brown-colored maghemite particles when exposed to a temperature of not less than 150.degree. C. In addition, the magnetite particles are apt to be re-coagulated with each other due to their magnetism, which makes it difficult for these particles to be dispersed in the binder resin, thereby causing deterioration in workability thereof. Also, in the case where such magnetite particles are used in coating materials to form coating films, the acid resistance of the obtained coating films are not necessarily satisfactory.
Carbon black has insufficient heat resistance and are in the form of bulky and ultra-fine particles having a particle size in the order of 0.01 to 0.1 .mu.m. As a result, it is difficult to handle the carbon black and therefore, its workability is deteriorated. Also, the carbon black has posed problems concerning safety and hygiene because it has been considered to be a carcinogen.
Graphite fluoride has a problem concerning safety and cannot show sufficient heat resistance because it undergoes undesired discoloration even when exposed to a temperature in the order of 500.degree. C. Further, the graphite fluoride has an insufficient dispersibility.
On the other hand, in association with the recent tendency that video and audio magnetic recording and reproducing apparatuses are reduced in size and weight, and are increased in recording time, there has been caused an increasing demand for magnetic recording media such as magnetic tapes or magnetic disks having a high performance, i.e., high recording density, high durability, good signal recording property or the like.
Since the magnetic recording media such as magnetic tapes or magnetic disks are brought into contact with a magnetic head upon recording and reading-out thereof, the surface of the magnetic recording medium is likely to be abraded into powdery dusts, thereby causing contamination of the magnetic head and further deterioration in reading-out properties thereof. For this reason, there have hitherto been demanded magnetic recording media having less abrasion and high durability.
Conventionally, in order to enhance an abrasion resistance and durability of the magnetic layer in the magnetic recording medium, it has been attempted to add various fillers such as alumina (Al.sub.2 O.sub.3), hematite (.alpha.-Fe.sub.2 O.sub.3) and chromium trioxide (Cr.sub.2 O.sub.3) in the magnetic layer.
For example, as conventional magnetic recording media using alumina (Al.sub.2 O.sub.3) as the filler, there are known a magnetic recording medium using .alpha.-Al.sub.2 O.sub.3 having an amorphous phase (Japanese Patent Application Laid-open (KOKAI) No. 5-36059(1993)), a magnetic recording medium using .alpha.-Al.sub.2 O.sub.3 having a specific crystal structure (Japanese Patent Application Laid-open (KOKAI) No. 7-244836(1995)), and the like. As conventional magnetic recording media using hematite (.alpha.-Fe.sub.2 O.sub.3) as the filler, there are known a magnetic recording medium using granular .alpha.-Fe.sub.2 O.sub.3 (Japanese Patent Application Laid-open (KOKAI) No. 61-194628(1986)), a magnetic recording medium using liquid hydrocarbon and .alpha.-Fe.sub.2 O.sub.3 (Japanese Patent Application Laid-open (KOKAI) No. 54-70806(1979)), and the like. Further, as conventional magnetic recording media using chromium trioxide (Cr.sub.2 O.sub.3) as the filler, there are known a magnetic recording medium using acicular Cr.sub.2 O.sub.3 (Japanese Patent Application Laid-open (KOKAI) No. 62-112221(1987)), and the like.
However, these fillers have posed individual problems. For example, it is known that alumina has a deteriorated dispersibility in the binder resin. Therefore, as the amount of alumina added to the binder resin is increased, the signal recording property of the magnetic recording medium is considerably decreased. Although hematite has a relatively good dispersibility in the binder resin, in order to obtain a sufficient durability of the magnetic recording medium, a large amount of hematite particles must be added to the binder resin. This causes the decrease in pack density of magnetic particles, thereby resulting in deteriorated signal recording property of the magnetic recording medium. Also, chromium trioxide is unfavorable from environmental and sanitary viewpoints.
That is, it is generally known that if the amount of such a filler added is increased to obtain a sufficient durability of the magnetic recording medium, the signal recording property thereof is considerably deteriorated.
For these reasons, there have been demanded excellent fillers for magnetic recording media which do not cause deterioration in signal recording property of the magnetic recording medium even though the fillers are used in an amount enough to improve the durability thereof.
At present, video tape systems generally used are provided with a mechanism for detecting an end position of a magnetic tape. In such a mechanism, a transparent leader tape located at an end of the magnetic tape is detected by a sensor. Accordingly, it is required that a magnetic recording portion of the magnetic tape has a low light-transmittance, i.e., a high degree of blackness.
However, in recent years, in order to meet requirements for high-density recording, magnetic particles used for the magnetic recording medium are considerably reduced in size. This results in increasing a light transmittance of a magnetic recording layer formed in the magnetic recording medium. As a result, there has been a risk that the mechanism for detecting an end position of the magnetic tape cannot be accurately operated. In order to eliminate the afore-mentioned defects, there has been adopted such a method that the content of carbon black particles in the magnetic layer is increased to lower the light transmittance thereof. However, since the carbon black particles have a low dispersibility in the binder resin, the addition of the increased amount of such carbon black particles causes the deterioration in dispersibility of magnetic particles. As a result, there are caused adverse influences on signal recording property and durability of the magnetic recording medium. In this regard, Japanese Patent Application Laid-open (KOKAI) No. 4-139619 (1992) describes " . . . when carbon black particles are added to the coating composition in the course of preparing the composition by mixing and kneading the binder resin and the magnetic particles, there arises a problem that the orientation and pack density of the magnetic particles are deteriorated as described later in Comparative Examples. In addition, since carbon black are in the form of bulky powder having a bulk density in the order of 0.1 g/cm.sup.3, the carbon black is difficult to handle and therefore, causes deteriorated workability. Further, the carbon black has problems concerning safety and hygiene such as carcinogenesis . . . ".
Under these circumstances, there has been a demand for adequate filler materials suitable as substituents for carbon black. However, the afore-mentioned fillers such as alumina, hematite or chromium trioxide cannot provide sufficiently low light transmittance as compared to the carbon black, because alumina has a white color, hematite has a red color and chromium trioxide has a green color. As conventional magnetic recording media using the black pigments as the filler, there are known a magnetic recording medium using black titanium (TiO) (Japanese Patent Publications (KOKOKU) Nos. 62-21185 (1987) and 62-22179 (1987)), a magnetic recording medium using graphite fluoride (Japanese Patent Application Laid-open (KOKAI) No. 56-156930 (1981)), or the like.
However, the black titanium is susceptible to oxidation and therefore unstable in air. The graphite fluoride shows a deteriorated dispersibility in the binder resin, thereby deteriorating the signal recording property of the magnetic recording medium.
For these reasons, there is a further demand for a magnetic recording medium using such a filler which has a good dispersibility in the binder resin and a high degree of blackness, which magnetic recording medium is free from deterioration in signal recording property of the magnetic recording medium even though the filler is added thereto in an amount required to obtain a sufficient durability thereof.
Incidentally, as safe and non-noxious non-magnetic black pigments, there are known Mn-containing hematite particles as disclosed in Japanese Patent Publications (KOKOKU) Nos. 43-17288 (1968) and 47-30085 (1973)), Japanese Patent Applications Laid-open (KOKAI) Nos. 49-124127 (1974), 51-149200 (1976) and 8-143316 (1996), or the like.
Further, there have been the following several prior publications disclosing the Mn-containing hematite particles. For example, Japanese Patent Application Laid-open (KOKAI) No. 52-63199 (1977) describes a method for producing a thermostable pigment by calcining a homogeneous mixture of ferrous sulfate and a sulfate of Mn as a modifying element; Japanese Patent Publication (KOKOKU) No. 54-37004 (1979) describes a method for producing a dark-red iron oxide pigment by calcining at least one starting material selected from Fe.sub.2 O.sub.3, FeOOH and Fe.sub.3 O.sub.4 in the presence of a manganese compound, phosphoric acid or the like; Japanese Patent Application Laid-open (KOKAI) No. 4-144924 (1992) describes a method for producing hematite particles containing a solid solution with Mn, by preparing spherical or granular magnetite particles by a wet process, coating the magnetite particles with a manganese (Mn) compound, or with manganese (Mn) and iron (Fe) compounds, and then calcining the coated magnetite particles; Japanese Patent Application Laid-open (KOKAI) No. 5-221653 (1993) describes composite oxides of manganese and iron having a hematite structure; Japanese Patent Applications Laid-open (KOKAI) Nos. 6-263449 (1994) and 7-66020 (1995) describe a method for producing Mn-containing hematite particles by conducting a wet oxidation process in the presence of a manganese compound to prepare Mn-containing goethite particles, and then heat-dehydrating the Mn-containing goethite particles.
More specifically, Japanese Patent Publication (KOKOKU) No. 43-17288 (1968) describes a method for producing a thermostable manganese-containing iron oxide pigment having a single hematite structure, comprising oxidizing iron by an aromatic nitro compound in the presence of a ferrous salt solution and/or a concentrated solution of a hydrolytically decomposable salt, a solution of trivalent or tetravalent metal salt, a suspension of insoluble oxide, hydroxide or basic salt-containing metal, phosphoric acid or a lead compound, or a core crystal (nucleus) influenced by yellow, reddish brown or red iron hydroxide or iron oxide, characterized in that metallic manganese and/or a manganese compound are uniformly added to the process while maintaining the pH of the process at about 3.5 to about 6.5, if necessary under the addition of alkali thereto; and the obtained pigment mixture is washed, filtered and heated at 600.degree. C. to 800.degree. C.
Japanese Patent Application Laid-open (KOKAI) No. 51-149200 (1976) describes a method for producing thermostable black and brown pigments composed of a mixed phase of iron oxide and manganese oxide, comprising the steps of (a) mixing granular pyrolusite having a particle diameter of not more than 20 .mu.m with ferrous sulfate heptahydrate in a range of room temperature to 95.degree. C. so as to be brought into contact with each other; and (b) calcining a product obtained after subjecting the mixture to mixing, drying, pulverizing and screening processes, in a fluidized bed reactor operated at 800.degree. C. to 900.degree. C. in the presence of air.
Japanese Patent Application Laid-open (KOKAI) No. 8-143316 (1996) describes an iron oxide-based pigment having a hematite structure composed primarily of iron, which pigment is composed of no-sintered but sinterable composite oxide particles having no spinel structure when measured by X-ray diffraction; has an average particle size of 0.01 to 0.1 .mu.m; and contains not more than 50 atomic % of manganese based on iron in the form of a solid solution with iron.
Japanese Patent Application Laid-open (KOKAI) No. 6-263449 (1994) describes (i) non-magnetic blackish brown iron oxide hydroxide particles having a goethite structure, which have a major axis diameter of 0.03 to 0.5 .mu.m and an aspect ratio (major axis diameter/minor axis diameter) of 2 to 15, and whose aggregate thereof shows a hue represented by L* value of 30.0 to 0, a* value of 6.0 to 0 and b* value of 10.0 to -1.7 (wherein the L* value, the a* value and the b* value represent values prescribed according to JIS Z 8729); and (ii) a method for producing iron oxide hydroxide particles having a goethite structure by passing an oxygen-containing gas through a suspension containing iron-containing precipitate obtained by reacting an aqueous alkaline solution with an aqueous ferrous salt solution to conduct the oxidation reaction in the suspension, characterized in that said aqueous alkaline solution is composed of both an aqueous alkali hydroxide solution and an aqueous alkali carbonate solution; and the aqueous alkali carbonate solution is added to any one of the aqueous alkali hydroxide solution and the suspension containing iron-containing precipitate, wherein the amount of the aqueous alkali carbonate solution added is in the range of 0.4 to 20.0 moles based on one mole of the aqueous alkali hydroxide solution (except for the amount of the aqueous alkali hydroxide solution required for neutralizing a manganese compound added thereto), the sum of the aqueous alkaline solutions is more than 1.0 equivalent amount based on Fe.sup.2+ contained in the aqueous ferrous salt solution, and the manganese compound is preliminarily added in an amount of 1 to 50 atomic % (calculated as Mn) based on Fe.sup.2+ contained in the aqueous ferrous salt solution, to any one of the aqueous ferrous salt solution, the aqueous alkali hydroxide solution, the suspension containing the iron-containing precipitate before subjected to the oxidation reaction by passing the oxygen-containing gas through the suspension and the suspension containing the iron-containing precipitate having a degree of oxidation of less than 50% by passing the oxygen-containing gas through the suspension.
Japanese Patent Application Laid-open (KOKAI) No. 7-66020 (1995) describes (i) non-magnetic blackish brown iron oxide hydroxide particles having a goethite structure, which have a major axis diameter of 0.03 to 0.5 .mu.m and an aspect ratio (major axis diameter/minor axis diameter) of 10 to 30, and whose aggregate thereof shows a hue represented by L* value of 28.0 to 0, a* value of 5.3 to 0 and b* value of 9.0 to -1.7 (wherein the L* value, the a* value and the b* value represent values prescribed according to JIS Z 8729); and (ii) a method for producing iron oxide hydroxide particles having a goethite structure by passing an oxygen-containing gas through a suspension containing iron-containing precipitate obtained by reacting an aqueous alkaline solution with an aqueous ferrous salt solution to conduct the oxidation reaction in the suspension, characterized in that said aqueous alkaline solution is composed of both an aqueous alkali hydroxide solution and an aqueous alkali carbonate solution; and the aqueous alkali carbonate solution is added to any one of the aqueous alkali hydroxide solution and the suspension containing iron-containing precipitate, wherein the amount of the aqueous alkali carbonate solution added is in the range of 0.4 to 20.0 moles based on one mole of the aqueous alkali hydroxide solution (except for the amount of the aqueous alkali hydroxide solution required for neutralizing a manganese compound added thereto), the sum of the aqueous alkaline solutions is more than 1.0 equivalent amount based on Fe.sup.2+ contained in the aqueous ferrous salt solution, and ascorbic acid or a salt thereof and the manganese compound are preliminarily added to any one of the aqueous ferrous salt solution, the aqueous alkali hydroxide solution, the suspension containing the iron-containing precipitate before subjected to the oxidation reaction by passing the oxygen-containing gas through the suspension and the suspension containing the iron-containing precipitate having a degree of oxidation of less than 50% by passing the oxygen-containing gas through the suspension, the amount of the manganese compound added being in the range of 1 to 50 atomic % (calculated as Mn) based on Fe.sup.2+ contained in the aqueous ferrous salt solution.
Although the afore-mentioned known Mn-containing hematite particles are safe and non-noxious, they are unsatisfactory in blackness because their hue is reddish brown or blackish brown. In addition, the conventional hematite particles has insufficient dispersibility in the binder resin. Therefore, in the case where these particles are used to form coating films or rubber or resin compositions, the resultant products cannot have sufficient acid resistance and aging resistance.
Accordingly, at present, it is most required to provide non-magnetic black pigments which are excellent in blackness and heat resistance and can show an excellent dispersibility in the binder resin. However, such black pigments which can fulfill all of these requirements have not yet been obtained.
Also, in magnetic recording media using fillers such as alumina, hematite and chromium trioxide, when the amount of these fillers added is increased to obtain a sufficient durability, the signal recording property of the magnetic recording media is considerably deteriorated. Presently, magnetic recording media which can fulfill both the requirements for durability and signal recording property have not yet been available.
Accordingly, one of technical subjects of the present invention aims at providing a safe and non-noxious non-magnetic black pigment which is excellent not only in blackness, heat resistance but also in dispersibility in the binder resin; a non-magnetic black paint obtained by using the non-noxious non-magnetic black pigment, for forming a coating film having an excellent acid resistance; and a black rubber or resin composition obtained by using the non-noxious non-magnetic black pigment, which has excellent acid resistance and aging resistance.
Further another technical subject of the present invention aims at providing a non-magnetic black filler having an excellent dispersibility in the resin; and a magnetic recording medium obtained by using the non-magnetic black filler, which has an excellent durability and show a good signal recording property, by using black filler particles which are free from deterioration in signal recording property even though they are added in an amount enough to improve a durability of the magnetic recording media.
As a result of studies and investigations by the present inventors, it has been found that by wet-pulverizing a slurry containing Mn-containing non-magnetic black particles composed primarily of Fe and having a hematite structure and an average particle size of 0.1 to 10.0 .mu.m, adjusting the pH of the slurry to not less than 13, and then heat-treating the resultant slurry at not less than 80.degree. C., the thus obtained non-magnetic black particles have a high blackness, an excellent heat resistance and a good dispersibility in a vehicle. The present invention has been attained on the basis of this finding.